In recent years, a capacity of non-volatile semiconductor memory devices, such as flash memories, has significantly increased, and products having a capacity of approximately 4 gigabytes have been sold at a price of approximately several tens of thousands of yen. In particular, a commercial value of portable or mobile memories, such as USB memories, has been increasing in such a manner as to grab a market that had been occupied by magneto-optical discs. In addition, the capacity of several gigabytes is sufficient for the storage in portable music players. Portable music players equipped with a non-volatile semiconductor memory device that is a solid element successfully appeal to users as being fundamentally superior as a solid element memory with regards to resistance to vibration, reliability and power consumption over portable music players equipped with a hard disc that has been spreading rapidly, and thus, are expected to be a mainstream storage system for portable or mobile products for music and images as described above.
In the case where a further increase in the capacity and reduction in the cost per bit are realized in the future, it would be possible for the non-volatile semiconductor memory devices to be used as the storage system for portable or mobile products which records and replays videos, and therefore, the next generation non-volatile semiconductor memory devices have been researched. In particular, the next generation non-volatile semiconductor memory devices may substitute for DRAMs used as a main memory in current information apparatuses if they can maintain a low cost and small cell area (not more than 4F2:F is the minimum processing size in manufacturing process) which are advantages of flash memories and overcome the following restrictions due to the principle of operation of flash memories: (1) high voltage for program/erasure (requiring a booster circuit), (2) slow operation for program/erasure (in particular, time for erasure exceeding 100 μsec), and (3) the limited number of times memory can be written (less than 106 times). As a result, it becomes possible to implement a so-called “instant on computer” which starts up instantly at the time of use and of which the power consumption is infinitely close to zero during standby.
Although non-volatile memory elements based on their own principles, such as ferroelectric memories (FeRAM) and magnetic memories (MRAM) have been researched and developed as candidates for this next generation non-volatile semiconductor memory device, it is difficult to outperform flash memories in such features as low cost per byte and small cell area.
Under such circumstances, phase change memories (PRAM), resistance change memories (RRAM) and the like attract interest because there is a possibility that they may exceed the low cost per bit of the flash memories. The resistance change memories are referred to herein as non-volatile memory devices where the electrical resistance can be changed when a voltage (or a current) not less than a threshold voltage (or a threshold current) is applied to a variable resistor sandwiched by electrodes in such a manner that this state of resistance is maintained in a non-volatile manner even after the application of a voltage (or a current) is once released, and information corresponding to different states of resistance can be stored.
For example, the following Patent Document 1 and Non-Patent Document 1 disclose “methods for changing the resistance value by applying a voltage pulse having a different polarity to a thin film made of a perovskite substance sandwiched between a pair of electrodes.” However, the perovskite substance has a problem in an affinity with general semiconductor processes, and therefore, a resistance change memory made of a binary oxide having a high affinity with semiconductor processes and a simple composition has been attracting interest, as disclosed in the following Patent Document 2. Patent Document 2 discloses a “non-volatile memory device characterized in that the data storing substance layer is a transition metal oxide film made of NiO, V2O5, ZnO, Nb2O5, TiO2, WO3 or CoO having different resistance properties for different voltages and a resistance which suddenly rises in a predetermined voltage range.” In fact, the following Non-Patent Document 2 reports an example of a non-volatile resistance change memory element where NiO, TiO2, ZrO2 or HfO2, which are binary transition metal oxides, is sandwiched between an upper electrode and a lower electrode. In addition, the following Non-Patent Document 3 discloses a resistance change memory element in such a form that two electrodes are electrically connected via a metal with which a hole penetrating through an insulating film is filled in.
Patent Document 1: U.S. Pat. No. 6,204,139
Patent Document 2: Japanese Unexamined Patent Publication No. 2004-363604
Non-Patent Document 1: Liu, S. Q. et al. “Electric-pulse-induced reversible Resistance change effect in magnetoresistive films,” Applied Physics Letter, Vol. 76, pp. 2749-2751, 2000
Non-Patent Document 2: Back, I. G. et al. “Highly Scalable Non-volatile Resistive Memory using Simple Binary Oxide Driven by Asymmetric Unipolar Voltage Pulses,” IEDM Technical Digest, pp. 587-590, 2004
Non-Patent Document 3: Ogimoto, Y. et al. “Resistance switching memory device with a nanoscale confined path,” Applied Physics Letter, Vol. 90, 143515, 2007